The present invention relates to a fuel cell system, which includes a fuel cell having a cathode input and a cathode output, a cathode supply path situated upstream from the cathode input and connected thereto, a cathode exhaust gas path situated downstream from the cathode output and connected thereto, a conveying means situated in the cathode supply path for conveying a cathode gas flow into the cathode input and/or an adjustable exhaust gas throttle means situated in the cathode exhaust gas path for influencing a flow resistance of the cathode exhaust gas path, and a regulating device, configured to regulate the cathode gas flow and/or a cathode pressure. The present invention furthermore relates to a method for operating a fuel cell system of this type.
Fuel cells use the chemical conversion of a fuel into water with the aid of oxygen to generate electrical energy. For this purpose, fuel cells include the so-called membrane electrode assembly (MEA) as a core component, which is a structure made up of an ion-conducting (usually proton-conducting) membrane and a catalytic electrode (anode and cathode) situated on both sides of the membrane. The latter includes usually supported noble metals, in particular platinum. In addition, gas diffusion layers (GDL) may be situated on both sides of the membrane electrode assembly, on the sides of the electrodes facing away from the membrane. The fuel cell is generally formed by a large number of MEAs situated in a stack, whose electrical powers add up. Bipolar plates (also referred to as flow field or separator plates) are situated between the individual membrane electrode assemblies and ensure a supply of operating agents, i.e., reactants, to the individual cells and are usually also used for cooling. In addition, the bipolar plates ensure an electrically conductive contact to the membrane electrode assemblies.
During the operation of the fuel cell, the fuel (anode operating medium), in particular hydrogen H2 or a hydrogen-containing gas mixture, is supplied via a flow field of the bipolar plate, which is open on the anode side, to the anode, where an electrochemical oxidation of H2 to protons H+ takes place with the discharge of electrons (H2→2 H++2 e−). A (water-bound or water-free) transfer of protons from the anode space into the cathode space takes place via the electrolyte or the membrane, which separates and electrically insulates the reaction spaces from each other in a gas-tight manner. The electrons provided at the anode are supplied to the cathode via an electric line. Oxygen or an oxygen-containing gas mixture (for example air) is supplied as the cathode operating medium to the cathode via a flow field of the bipolar plate, which is open on the cathode side, so that a reduction from O2 to 2 O2− takes place with the absorption of the electrons (½O2+2 e−→O2−). At the same time, in the cathode space, the oxygen anions react with the protons transferred via the membrane, forming water (O2−+2 H+→H2O).
To supply a fuel cell stack with its operating media, i.e., the reactants, the fuel cell stack includes an anode supply system on the one hand and a cathode supply system on the other hand. The anode supply system includes an anode supply path for supplying an anode operating gas to the anode spaces and an anode exhaust gas path for removing an anode exhaust gas from the anode spaces. Likewise, the cathode supply system includes a cathode supply path for supplying a cathode operating gas to the cathode spaces and a cathode exhaust gas path for removing a cathode exhaust gas from the cathode spaces of the fuel cell stack.
A number of peripheral components (ancillary units) are required to operate a fuel cell stack. Among other things, they may include air conveying means for conveying a cathode gas flow into the cathode (for example an air compressor), a recirculation blower, a cooling water pump, throttle means for influencing flow resistances (for example valves), sensors, etc. The power consumption of these components is referred to as parasitic consumption, since this energy must be provided by the fuel cell stack but is not available to external consumers. Since the current available to external consumers is reduced by the parasitic current, the total efficiency of fuel cell system ηSys is always less than the efficiency of fuel cell stack ηFC.
During the operation of a fuel cell, a load-dependent regulation of an air ratio lambda (also known as cathode lambda) takes place, which correlates to an (air) mass flow into the cathode (also known as cathode mass flow), and a boost pressure of a fuel cell (also known as cathode pressure). The boost pressure is the pressure via which the air compressor supplies the cathode operating gas to the cathode.
DE 11 2005 000 767 T5 describes another fuel cell system, which includes an air compressor situated upstream from the cathode, which is driven by a motor so that the air is introduced according to the rotational speed of the motor. The fuel cell system furthermore includes a pressure control valve situated downstream from the cathode, which is controlled in such a way that the pressure is adapted to the air to be supplied to the fuel cell stack. The fuel cell system also includes a recirculation valve, which establishes a connection of the cathode lines upstream from the air compressor and downstream from the pressure control valve. A quantity of humidified air supplied to the fuel cell stack is controlled via the recirculation valve. If it is determined during the operation of the fuel cell system that the flow generation quantity of the fuel cell stack has been increased, the air supply quantity is increased by increasing the rotational speed of the motor of the air compressor. Due to the increase in the rotational speed, the flow rate of the supplied air increases, and the pressures within the fuel cell stack also increase. A pressure sensor detects this increased pressure. On this basis, a control takes place to reduce the pressure by increasing an opening degree of the pressure control valve for the purpose of keeping the pressure constant within the fuel cell stack. If it is determined that the electrical current generated by the fuel cell stack has been reduced during the operation of the fuel cell system, the rotational speed of the motor of the air compressor is reduced. The pressure decreases along with the decreasing rotational speed of the air compressor. The control to increase the pressure value is executed on the basis of the pressure value of the pressure sensor.
Arendt, M; Regelungstechnische Optimierung der Steuerung eines Brennstoffzellensystems im dynamischen Betrieb (Optimizing the Control of a Fuel Cell System During Dynamic Operation), Dissertation, Logos Verlag Berlin, 2012, discloses a fuel cell system which includes a fuel cell stack. The fuel cell system furthermore has a cathode supply path and a cathode exhaust gas path. The cathode supply path includes an electrical compressor, a mass flow sensor situated upstream from the compressor, and a pressure sensor at an inlet of the fuel cell stack. The cathode exhaust gas path, i.e., an exhaust gas line of the fuel cell system, has an adjustable throttle element (a throttle valve). The fuel cell system furthermore has a waste path (also referred to as a waste line). The waste path connects the cathode supply path upstream from the conveying means to the cathode exhaust gas path downstream from the exhaust gas throttle means and includes another adjustable throttle element (a waste valve). The waste path also includes a mass flow sensor. An opening of the throttle element in the waste path and a rotational speed of the conveying means (air compressor) are used as manipulated variables to regulate an air mass flow through the fuel cell stack and an air mass flow through the compressor (control variables). To regulate the air pressure at the cathode inlet (i.e., the cathode pressure), it is proposed to use an opening of the throttle element in the cathode exhaust gas path as a manipulated variable.
DE 10 2008 039 407 A1 discloses a similar system structure. Its fuel cell system differs from the above fuel cell system in that the fuel cell system includes a compressor bypass, which branches off in the cathode supply path downstream from the electrically operable compressor and opens into the cathode supply path (into a compressor suction system) upstream from the compressor. The compressor bypass thus does not lead to the surroundings. A mass flow sensor may also be present in the compressor bypass. The compressor bypass furthermore includes a valve element. An opening of the valve element in the compressor bypass and a rotational speed of the electrically operable compressor are used as manipulated variables for setting the air quantity. Once again, a throttle valve is provided in the cathode exhaust gas path to be able to vary the pressure level in the system. The pressure level in the fuel cell may be additionally influenced in this way by changing a flow resistance with the aid of the throttle valve.
Setpoint values for the boost pressure and the cathode lambda (i.e., the cathode mass flow) are generally predefined as a function of the load, i.e., as a function of a load of the fuel cell. The setpoint values may be determined, for example, from characteristic lines. However, the cathode pressure and cathode mass flow typically increase as the load demand increases. A setpoint boost pressure is coupled to an anode pressure (within the anode of the fuel cell). The anode pressure may usually be dynamically adjusted on the basis of a high dynamics of a corresponding component.
In low load ranges or during transient operation, for example downward-transient due to the coupling of the anode and cathode pressures, the system characteristic line of the fuel cell may be outside or in the vicinity of a pump limit of the compressor. This is particularly significant in turbocompressors. If the pump limit is exceeded, a compressor pumping action may set in, which may cause damage to the compressor and must therefore be avoided. In critical operating states, therefore, the (setpoint) air mass flow through the compressor is increased, and the operating point of the compressor is shifted thereby into stable ranges. To ensure a setpoint value sequence (for example, due to a moisture management system in the stack, etc.), the additional air mass flow is blown into the cathode exhaust gas path via the waste path or into the compressor suction system via the bypass path.
A transient operation refers to a change in the operating point of the fuel cell system. An upward-transient operation thus refers to an increase in the load demand on the fuel cell and an associated increase in the cathode mass flow and the cathode pressure. A downward-transient operation thus refers to a reduction in a load demand on the fuel cell and an associated reduction in the cathode mass flow and the cathode pressure.
Numerous disadvantages now result from the known prior art. The electrical compressor is usually the biggest secondary consumer within a fuel cell system, which is why it is a good idea to minimize the power consumption of the electrical compressor. The power consumption may be reduced by removing a preferably small mass flow via the waste path. This must take place while simultaneously maintaining a predefined cathode mass flow and cathode pressure to ensure a good stack efficiency as well as a sufficient service life (read: moisture management) or only a minor degradation of the fuel cell.
A regulation using the above assignment of manipulated values to the control values tends to become unstable, since small changes in the rotational speed of the compressor in the operating range under consideration induce large changes in the mass flow.
Moreover, hot film air mass meters (HFM) are typically used as the air mass meter. If the compressor rotational speed is used as a manipulated variable, as described above, to regulate the cathode mass flow, a measurement noise of the hot film air mass meter causes the power consumption of the compressor to oscillate. This, in turn, results in problems in the power electronics of the compressor.
It is furthermore known that, in some compressors, two possible mass flows of different sizes through the compressor may set in at a constant pressure ratio and a constant rotational speed of the compressor. This results in the fact that there is no clear way to adjust the mass flow as a function of the turbocharger rotational speed and the pressure ratio (formed from the compressor input pressure and compressor output pressure).